Is Robinson Arithmetic complete and not-complete?

Question

Is Robinson Arithmetic complete in the sense of Gödels completeness theorem? And is Robinson Arithmetic incomplete in the sense of Gödels first incompleteness theorem? If RA is both it would be a good example to explain the different notions of "completeness" used in the two theorems mentioned.

Answer 1

Gödel's completeness theorem isn't about theories but about the logic used in the language of a theory, namely about the connection between the syntax and semantics of that logic. The logic used to describe Robinson's arithmetic (first order logic) is complete and at the same time RA is not complete as a theory. The same applies to first order Peano Arithmetic. The logic used in the language of PA is complete but PA is not a complete theory. Same thing applies to ZFC. Generally, every incomplete theory described in first order logic has that property.